This research proposes to examine the contribution of genetic factors in determining hemispheric specialization for speech perception. The problem will be approached by assessing cerebral organization in monozygotic and dizygotic twin pairs and subsequently comparing the degree of similarity within pairs on this dimension for each group. The dichotic listening pardigm will be used to provide a measure of hemispheric specialization for speech for the analysis. Greater within pair similarity in the monozygotic twins than in the dizygotic twins will provide evidence for the contribution of genetic factors in the determination of cerebral organization. Monozygotic twins showing signs of mirror imaging will be analyzed separately to determine if mirror imaging effects may extend to cerebral organization.